Co(Ti1-xZrx)Nb2O8 microwave dielectric ceramics were synthesized via the conventional solid-state reaction route. The dependence of microwave dielectric properties on the crystal structure was discussed. The phase transitions were analyzed using X-ray powder diffraction, Raman spectroscopy, and transmission electron microscopy. A series of composition-induced phase transitions were confirmed via the sequence: tetragonal rutile structure → coexistence of rutile and wolframite phase → monoclinic wolframite structure. For the Co(Ti1-xZrx)Nb2O8 compounds, Zr substitutions from 0 to 1 led to a decrease in the εr from 62.4 to 23.8. In contrast to εr, Q × f increased considerably, which was explained in terms of packing fraction. The τf was correlated with oxygen octahedral distortion and B-site bond valence. A near zero τf of 4.4 ppm/°C was obtained in the CoTi0.4Zr0.6Nb2O8 ceramics with an εr of 29.9 and a high Q × f of 72,833 GHz.

Low-fired CuO-doped CoTiNb2O8 microwave dielectric ceramics were synthesized via a conventional solid-state reaction method and the effects of CuO additives on their sintering behavior, phase composition, microstructure and microwave dielectric properties were investigated systematically. The addition of CuO to CoTiNb2O8 successfully lowered the sintering temperature from 1250 to 950 °C. Liquid phase CuNb2O6 was formed due to the chemical reaction between CuO and matrix for the CuO-containing samples; meanwhile, (Co, Cu)TiNb2O8 solid solution was detected by XRD and EDS analysis. Both of them promoted the densification of CoTiNb2O8 ceramics. The addition of CuO did not induce apparent degradation in microwave properties but tailored the τf values toward zero direction. With 2 wt% CuO, CoTiNb2O8 ceramics sintered at 950 °C reached the maximum relative density of 98.21% and possessed optimum microwave dielectric properties with εr of 61.45, a high Q×f of 15,938 GHz, and a τf of 42.12 ppm/°C, making them a promising candidate for LTCC applications.

•New CoTiNb2O8 ceramics were prepared via sol-gel method.•Well-crystallized target powders with average grain size of 34 nm were obtained at 800 °C.•Typical values of εr∼64.19, Q × f∼16,800 GHz and τf∼66.17 ppm/°C were achieved.CoTiNb2O8 microwave dielectric ceramics were synthesized via a modified sol-gel route. The thermal decomposition behavior of the as-prepared precursor was characterized by differential thermal analysis (DTA)/thermogravimetry (TG). Highly reactive CoTiNb2O8 nano-powders with average particle size of 34 nm were achieved by the reaction between TiO2 and intermediate phase CoNb2O6 at 800 °C. The microwave dielectric properties of ceramics were sensitive to processing condition and crystal structure. For the CoTiNb2O8 compounds, the variations of εr and Q × f value agreed well with that of the relative density. The τf was explained in terms of oxygen octahedral distortion and B-site bond valence. At 1000°C/4h, CoTiNb2O8 ceramic possessed optimal microwave dielectric properties with an εr of 64.19, a high Q × f of 16,800 GHz (at 6.4 GHz) and a τf of 66.17 ppm/°C.

Single phase of Bi2Ti4O11 ceramics, which belong to meta-stable phase compounds, were synthesized by controlling the reaction time through conventional solid-state method. The effects of annealing time on phase composition of Bi2Ti4O11 ceramic powders and sintered ceramics were studied by XRD analysis. Second phase Bi2Ti2O7 appeared when the annealing time shorter than 4 h. However, pure phase of Bi2Ti4O11 powders can be formed by prolonging the annealing time to 6 h at 1,000 °C. The sintering temperatures on microstructure and microwave dielectric properties of Bi2Ti4O11 ceramics were investigated. The results show that ceramics sintered at 1,075–1,175 °C are single phase of Bi2Ti4O11 and present two different sizes of prismatic shape grains. Smaller size crystals grow into larger ones with increasing sintering temperature. The ceramics sintered at 1,125 °C reach a maximum density and have a microwave dielectric properties of εr = 51.2, Q × f = 3,050 GHz and τf = −297 ppm/°C.

ZnTa2O6 microwave dielectric ceramics have been prepared using ZnTa2O6 nano-powders synthesized by sol–gel processing in this study. The crystal structure and microstructure of the ZnTa2O6 powders and ceramics were characterized by XRD and SEM techniques. ZnTa2O6 ceramics can be densified at a lower sintering temperature of 1200 °C. Microwave dielectric properties show that both of Q × f and ɛr values are lower than those of ceramics prepared by solid state route, and the τf values do not show different from that of solid state route. ZnTa2O6 ceramics sintered at 1200 °C exhibit good microwave dielectric properties: Q × f = 50,600 GHz, ɛr = 35.12 and τf = 9.69 ppm/°C.

A sol–gel processing was used to synthesize MgTa2O6 nano-powders at a low temperature using Ta2O5 as starting materials. The decomposition of precursors, and the crystal structure and microstructure of the MgTa2O6 powders were characterized by DTA/TG, XRD and SEM techniques. The effects of the amount of citrate acid and pH values on the stability of sol–gel solution and microstructure of MgTa2O6 nano-powders were investigated. XRD and TG/DTA results show that the MgTa2O6 nano-powders can be synthesized at 900 °C, and non-stoichiometric ratio of Mg to Ta was required to form the single MgTa2O6 phase. The average particle sizes of MgTa2O6 powders range from 25 nm to 100 nm with different CA/Ta and pH values.